This invention relates to a lost foam process for casting aluminum that comprises an in-mold treatment with a halide salt flux to inhibit formation of oxide fold defects in the cast product. More particularly, this invention relates to an aluminum lost foam casting process that incorporates a body of the halide flux into an expendable pattern for distribution onto a front of the cast metal as the front progressively decomposes the pattern during casting.
In a typical lost foam casting process, a vaporizable pattern formed of steam-fused polystyrene beads is embedded in a mold of unbonded refractory particles. Molten metal poured into the mold progressively decomposes and replaces the pattern.
It has been found that aluminum castings produced by a lost foam process may have a defect characterized by a thin aluminum oxide film within the body of cast metal. Although microscopically thin, the film may extend over an area as great as several hundred square millimeters. Based upon its appearance, it is postulated that the defect may be formed by a contaminated metal surface that becomes folded into the body of metal during casting, perhaps as the result of collapse of a melt front exposed to the decomposing pattern and oxidized by residual water therein. The fold typically contains carbon from the decomposing pattern, which has a noticeable black appearance. However, the problems associated with folds are mainly attributed to the tenacious oxide. The fold forms a sheet-like discontinuity in the body of metal, similar to particulate inclusions but having considerably greater area. This may substantially weaken a cross-section of the casting and lead to catastrophic fracturing. Also, fissures that form at the fold may produce a leak that may render the casting unsuitable for an intended hermetic use.
Oxide folds are extremely difficult to detect except after fracturing, since the film is mainly buried within the casting and forms at most only a microscopic line at the surface. Comparable defects are not reported for castings formed within a cavity, such as die castings. However, elimination of oxide fold defects has been a major concern in lost foam casting.
Therefore, it is an object of this invention to provide a treatment to improve soundness of an aluminum casting formed by a lost foam process, which treatment comprises suitably dispersing onto a metal surface adjacent the decomposing pattern within the mold a halide agent effective to inhibit formation of a tenacious oxide film on such surfaces and thereby decrease oxide fold defects in the product.